Amateur radio (also called ham radio) describes the use of radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communication.

A GUIDE TO ANTENNA SELECTIONBY ANDREW CORPORATION

INTRODUCTION

Since the introduction and practical application of Log Periodic and
frequency independent antennas, a wide range of truly broadband antennas
has been developed to meet the varying demands imposed by specific
requirements, such as area coverage and point-to-point circuits. The
development of compatible terminal equipment such as baluns, impedance
transformers and multicouplers has further enhanced the flexibility of
these broadband radiating structures.

Choosing the right type of antenna for use on a HF link is very
important if the best link performance is to be achieved. The choice of
antenna types is large and, at times, baffling. A number of antennas
that will more or less satisfy the requirements in question are
available, finding the optimal solution is far from simple for the
systems planner. It requires a knowledge of iononspheric behavior as
well as antenna engineering, operating conditions and siting
considerations. Because of this, the systems planner usually enlists the
help of the antenna specialist to analyze the propagation conditions
and find (or design, if necessary) an antenna to fit the requirement. In
essence, the antenna specialist, to carry out this function
effectively, needs to be a physicist, applications engineer, structural
engineer and applied mathematician. This bulletin sets out to dispel
that myth, explain how antenna types vary in performance and how these
differences can be most effectively exploited.

BASIC PARAMETERS

To differentiate between the various antennas available, an
understanding not only of the basic parameters of antenna performance,
but the different ways in which this information is presented by various
manufacturers is required.

The following notes are not intended as definitions but rather as
comments which may be helpful when examining manufacturer's technical
bulletins.

The Radiation Pattern of an antenna indicates the power (or field
strength) radiated in any direction relative to that in the direction
of maximum radiation. Both relative power and relative field diagrams
are in use and often no clear statement is made as to which is
presented.

The actual radiation pattern of any antenna is really a
three-dimensional function, however, for the sake of simplicity, only
cuts through this solid in the horizontal (azimuth) and vertical
(elevation) plane as usually presented. These planes are referred to as
the principle planes. For one important class of antennas, those used on
short range paths via the ionosphere (i.e., vertical incidence), this
practice is not observed for the following reasons.

When considering the behavior of an antenna operating over a short path
and consequently required to direct energy at high angles of elevation,
the conventional principle plane radiation pattern would be misleading,
in so far that the normal "azimuth radiation pattern" will only indicate
the variation in field strength at 90° to the zenith, a circle in the
horizontal plane of the antenna, and not that being reflected at the
ionosphere. A more useful set of data is obtained by traversing a path
which joins all points with constant elevation angle and lie equidistant
from the antenna. The radiation pattern so defined is always circular
at the zenith as this is the trivial case, and the familiar patterns
will be obtained normal to this. Patterns for a variety of elevation
angles from 50° upwards are commonly published. It should be noted that
the sense of polarization of the radiated field changes with bearing,
therefore, the polarization direction of the antenna for such operations
is not usually quoted, it is meaningless.

The radiation pattern in the elevation plane is strongly influenced by
the presence of the ground beneath the antenna as the radiated signal is
the sum of the signal radiated directly from the antenna and that
reflected by the ground. The relative phase of these components changes
with the antenna's height above ground, electrical properties of the
ground and polarization, either adding to or cancelling the field due to
the direct ray. Manufacturers' data generally refer to "average ground"
usually taken to have a conductivity of 10m S/m and a relative
permittivity of 10. These figures are typical of ground where grass or
agricultural crops are growing. Data quoted for "ideal ground" should be
viewed with reservation because, of course, no such ground exists,
except perhaps sea water, and an extensive earthment of copper wires
will be required to obtain an approximation to the published
performance.

A further characteristic of radiation pattern performance often quoted is the beamwidth, or properly the half power beamwidth.
This is the angle between the points on either side of the direction of
maximum radiation at which the intensity of power radiated has fallen
to half the maximum. Care is necessary as in some cases only half this
angle is quoted. Specifying the beamwidth as ± degrees removes any
ambiguity, unfortunately this is not always the case.

Power gain indicates how much the signal radiated in the
direction of maximum radiation is increased over and above that that
would be obtained if the antenna referred to is substituted for some
standard reference antenna fed with the same input power. Here again
there are possibilities for confusion. The reference antenna may be
either a half wave dipole (usually used for VHF and UHF antennas) or an
isotropic radiator (in the case of HF and SHF antennas). The isotropic
radiator is a hypothetical device radiating energy uniformly in all
directions in 3-dimensional space. As the power gain of an isolated
halfwave dipole is 2.2 dB over that of an isotropic radiator, it is
important to know which reference is used. Reference to an isotropic
radiator is normally indicated by ndBi. Quoted gains normally allow for
the enhancement of signal provided by ground reflection, but again the
assumed ground conditions are often omitted.

Maximum input power will generally be determined by the onset of one of three effects:

Dielectric losses causing overheating of insulators.

Ohmic losses as a result of conductors carrying large currents.

Corona discharge from insulators, element tips or other parts.

As some of these effects are current determined and others voltage
determined, the ratio of power ratings quoted for different classes of
emission will vary. Take care if transmitters are to be parallel
operated into one antenna as very high peak voltages may be produced.

Input Impedance. As the transmission line feeding an antenna must
have the same characteristic impedance as the antenna input impedance,
it is often the economics and practicalities of transmission line design
which determine suitable antenna input impedances. A wide range of
efficient broadband transformers are available allowing changes in
impedance level and from balanced to unbalanced line systems.

The economic need to minimize the size of the transmission line
required, especially when large diameter semi-flexible coaxial cable is
used. The power rating of a cable reduces approximately 1/o where o is
the VSWR.

The ability of the transmitter output circuit to match a non-optimal impedance.

In practice, condition 2 often dominates. Naturally, wide-band antennas
provide most problems in respect of VSWR as narrow band systems may
always be matched on site with little difficulty.

Receiving systems seldom require a closely specified VSWR. The VSWR data
should be taken from actual antennas of the generic type when correctly
installed under normal site conditions.

Ground Type-Average soil as defined by CCIR

Ground Constants-Conductivity 10 m S/m; Permitivity 10

Flatness-±1 metre

Slope-Nil

Obstructions-Presence of other antennas, towers, power lines,
metallic structures, etc., at a distance having negligible mutual
coupling effect

Reference Point-Antenna input (ground level)

VSWR can be quoted in two ways:

Nominal-Signifies a value not exceeded throughout 90% of the specified frequency range.

Max. or Peak-Signifies value not exceeded throughout worst 10% of frequency range.

Polarization. This parameter describes the direction of the
electric field vector of a propagating electro magnetic wave. When
referring to a directional antenna, it generally describes the
polarization radiated or received in the direction of the radiation
pattern maximum. Often signals of other polarizations are radiated in
other directions. At low angles of radiation, this can simply be defined
as vertical polarization being normal to the earth's surface and
horizontal being that which is parallel. However, at zenith both are
parallel, forming a cross directly above the point of reference.

ANTENNA SPECIFICATIONS

In order to select the optimum antenna for a given HF communication
requirement, it is necessary to establish a number of important
parameters; e.g.:

In addition to the above listed requirements, such matters as ease of
installation, maintenance requirements, ease of maintenance, method of
assembly, packaging and their effect upon transportation costs must
enter into the decision of antenna selection.

It has already been pointed out that the performance of a HF antenna is
dependent on the ground over which it is operating. If the electrical
parameters of the soil (conductivity and permitivity) are known, these
should be specified.

Having defined the basic requirements, we shall now consider the system
requirements and methods for establishing specifications and their
relationship.

Frequency Range. For predictions of usable frequencies reference
can be made to a number of sources; e.g., monthly ionospheric
predictions which give maps of the world for various times of the day on
which contours of the maximum usable frequencies (MUF) are
superimposed. By properly interpreting these charts which apply to the
particular location and times of interest, the usable frequencies can be
predicted. This method provides a long range average prediction and,
although variations in actual conditions occur from day to day, they are
useful for preliminary circuit planning. With the increased computing
capacity of personal computers (PCs), programs such as IONCAP are now
readily available to the antenna specialist. Such ionospheric models can
predict circuit behavior statistically by taking into account a great
many factors; where the old monthly charts gave a smoothed average of a
relatively small number of observations, they are able to accommodate
many more possible circuits. Taking due cognizance of such factors as
sunspot activity and the behavior of the E and F layers of the
ionosphere, including sporadic modes has increased our ability to
predict the effects of seasonal and diurnal fluctuations in the
ionosphere. The inclusion of ground reflectivity, noise levels,
signal-to-noise ratios and circuit reliability further enhances the
accuracy.

HF operation at 30° north latitude has been found to be reasonably
indicative of operation throughout the northern hemisphere, with the
exception of the region near the polar caps. Figure 1 shows the extreme
values of Frequency of Optimum Traffic (FOT) for paths centered at 30°
north latitude as a function of path length. This chart and the General
Propagation Chart, Figure 2, provide the antenna designer with an
approximation which serves as a useful guide in selecting a suitable
antenna. To use the chart, a line from the desired path length is drawn
down from the minimum and maximum FOT scales and across from the minimum
layer height scales. The region enclosed by the rectangle on the chart
determines the maximum frequency range and take-off angles for the
circuit. As an example, assume a 1000 km point-to-point circuit. The
frequency range (3.7 to 16.5 MHz) is obtained by selecting the
corresponding frequency interval between 1000 km on the minimum FOT
scale and 1000 km on the maximum FOT scale. The required take-off angle
range (24° to 40°) is found by selecting the corresponding take-off
angle interval between 1000 km on the 240 km layer height scale and 1000
km on the 450 km height scale.

Obviously it is generally desirable to incorporate maximum bandwidth
capability to prolong the operational lifetime of the antenna system.
The logarithmically periodic antenna and the application of the "angle
condition" (such as conical and equi-angular antennas) have been the
principle developments in the field of broadband high performance
antennas over the past few decades. In fact, the very meaning of
broadband changed with the introduction of frequency independent
antennas of which the log-periodic class has been the principle
embodiment. For this class of antennas, bandwidth limits are set by
practical, not theoretical, limitations. The low end of the band having
primary influence on the size of the antenna, dimensions of the active
portion of the antenna being comparable to the wavelength at the lowest
operating frequency; restrictions to the high end of the band are
usually set by fabrication techniques and tolerances consistent with the
structural requirements.

Polarization. When ionospheric paths are involved, the rotation
of polarization which occurs within the ionosphere generally has the
effect that the performance difference between vertical and horizontal
polarizations is negligible, providing that the effective gain of the
two antennas is identical. For transmitting, then, the antenna choice
should be made on the basis of the elevation pattern which provides the
highest effective gain at the expected take-off angles determined by
geometry, without regard to polarization. For receiving antennas, the
choice is complicated by an additional factor - the atmospheric noise
pick-up. For locally generated noise; i.e., man-made or natural static
arriving at the receiving site by ground-wave propagation, the noise
pick-up is almost always somewhat higher with vertical than with
horizontal polarization. For distant noise sources, the relative noise
pick-up depends on the effective antenna gain, not on the polarization,
as for any other signal source. The relative noise pick-up advantage of
horizontal polarization depends on many factors, most of which are
difficult to determine so that actual numbers are unavailable.

Horizontally polarized antennas are to some extent more versatile than
vertically polarized antennas, because the elevation plane radiation
pattern can be readily varied to suit the path requirements by changing
the height of the radiator above the ground plane. In general, if the
radiator is one-quarter wavelength or less above the ground, radiation
is essentially upwards, and raising the antenna further above the ground
tends to lower the radiation angle towards the horizon. The rapidly
increasing side-lobe level (in the elevation plane pattern) for radiator
heights greater than about one wavelength places a practical limit on
this, and use of horizontally polarized log-periodics are not generally
recommended where the nominal beam angle is less than 15°. Obviously the
horizontal log-periodics are most useful for short and medium range
circuits (requiring take-off angles in the order of 50° and 25°,
respectively).

Vertically polarized antennas, on the other hand, tend to have their
maximum radiation at lower angles in theory towards the horizon when the
ground is perfectly conducting. However, the earth is not perfectly
conducing and the ground parameters have considerable influence on the
actual radiation pattern of the antenna, but nevertheless, a vertically
polarized log-periodic with an adequate ground screen is found to be
best suited for propagation at low elevation angles.

One further fundamental difference of operational significance is that a
narrower beam is obtained in the principle plane which is parallel to
the dipoles or radiators of the array. Nature has a way of trading
beamwidth from one principle plane to another, so that the maximum gain
obtainable from an optimized array of horizontally dipoles is about the
same as that obtainable with an optimized array of vertical dipoles.

Elliptical polarization is a combination of the two fundamental planes,
the ellipticity being determined by the ratio of horizontal to vertical
components. If both were equal, the resultant wave would, in fact, be
circular. This mixed polarization minimizes the loss effects which
result due to the rotation of polarization which occurs within the
ionosphere. The overall advantages can be likened to that achieved
through polarization diversity where two antennas of opposite
polarization ensure the reception of the maximum available field.
Vertically polarized antennas cannot always provide reliable short range
coverage because of limitations in the radiation pattern around zenith
or excessive attenuation of the groundwave. Existing horizontally
polarized antennas are limited in capability to provide reliable long
range communications because of the lack of control of radiation
patterns over an adequate frequency range. However, communication
performance beyond that achieved with linear polarized antennas is
possible using elliptical polarization.

Elevation Plane Radiation Pattern. The matter of take-off angles
from the transmitting antenna and angle of arrival at the receiving
antenna is very important in selecting an antenna for any particular
circuit: so the natural question arises, "How do we know what this angle
is?" One approach is to make a scale drawing of the ray path which may
be done readily by adding distance and angular scales to diagrams of the
type illustrated by Figures 3a and 3b, sketching in the ray for a given
range and layer height and noting the elevation angle of the ray at the
antenna location for this particular path. We frequently use for this
purpose the Skywave Transmission Plot shown in Figure 4. The scales on
the chart indicates the distance along the surface between the antennas
or reflection points, the height of the reflection layer and the
take-off angle. A HF Antenna Selector is available which has the
elevation patterns for most antennas super-imposed onto Skywave
Transmission Plots (request Bulletin 1401.)

A simple example will illustrate the use of this chart. Suppose we are
concerned with a circuit of 1000 kilometres great circular distance. The
ionospheric reflection point will occur halfway between the stations,
and for F2 layer reflections the effective height may be assumed to
occur at about 300 km. By laying a straight edge on the chart (Figure 4)
between the antenna location (at the lower left corner) and this
assumed reflection point, the take-off angle can be read on the scale at
the top of the chart. In this case, the answer is 28°. In actual
operation, of course, the elevation angle of the signal path changes
from time to time as ionospheric conditions change. However, the usual
situation can be bracketed by assuming F2 layer reflections at about 300
km. On longer range circuits where multihop modes will occur, the
typical conditions can be obtained by following the procedures outlined
above for various sub-multiples of distance. If the vertical angle is
below 4°, repeat using an additional hop. The mode involving the least
number of reflections will almost always incur the lowest attenuation,
so this mode and perhaps the next one or two more complex ones will be
of greatest interest. Examples of multiple hop transmission for E and F
layer reflection can be found in Figure 5.

Figure 3b shows reflection of a signal from a lower layer in the
atmosphere - the E layer, which occurs at a much lower height (about 100
kilometres) and is active primarily during the day. The elevation angle
of the signal path is lower in this case than shown previously for
reflections from higher layers. In addition to these simple modes, many
more complex ones are possible on occasion, for example, those which
involve E layer reflections along one part of the path and F layer
reflections along the next. This might suggest that in antenna design
there is a problem of meeting a wide range of conditions. This is
certainly true, and a wide range of possible signal paths must be
accommodated in order to ensure reliable circuit performance over a long
period of time.

Azimuth Plane Beamwidth. Systems considerations that effect this
parameter are, of course, azimuth plane coverage requirements, such as
omnidirectional for some ground/air and shore/ship applications and
possible use of multicouplers in a broad band antenna whose elevation
pattern is suitable for simultaneous operation of various point-to-point
circuits. Other factors affecting azimuth beamwidth specifications are
gain, possible interference to (or from) other services, and off-great
circle propagation effects (which place a lower limit in the order of
10° to 15° to azimuth beamwidth).

Broadband antenna types with different azimuth plane coverage are
discussed below in the sections dealing with point-to-point and area
coverage circuits.

The spread in azimuth beamwidth is from about 60° to 110°.

Side Lobe Level and Front-to-Back Ratio. Ideally an antenna
should have no side lobes and infinite front-to-back ratio. This is, of
course, unrealistic so the question is what should be specified and what
can be attained. Firstly, however, it is useful to evaluate the effects
of secondary lobes upon system performance. They are:

Reduction in gain. Due account must be given to the power radiated
in the side lobes as it detracts from the directive gain of the main
beam. True directive gain of the antenna is established by integration
of the total power radiated.

Interference to (or from) other services. In this matter of
interference caused by antenna side lobes, consideration must be given
to individual applications, proper evaluation of propagation factors and
antenna radiation pattern must be made to determine susceptibility to
interference. This is particularly true with regard to side lobes in the
elevation plane. The section regarding antenna polarization points out
the dependence of elevation plane side lobe level upon height of a
horizontally polarized antenna above ground to achieve low take-off
angles. If elevation plane side lobes are a major consideration,
take-off angles less than about 20° should not be attempted with
horizontally polarized structures of the log-periodic type. With
log-periodic antennas of not undue complexity, side lobe levels in the
order of -12 dB are attainable.

Gain. It is important to note that when considering antennas for
HF communication systems, a distinction can sometimes be made between
transmitting and receiving applications with regard to the significant
definition of antenna gain. The distinction is that while the
transmitting antenna must meet a specification for power gain (which
includes in addition to directive gain a measure of the antenna
efficiency), the receiving antenna should only be required to meet a
specification for directive gain. This is because in most HF
communications situations, the system noise level is determined by
atmospheric noise.

Practical applications of this fact can result in a reduction in size of
the receiving antennas, however, the system planner must ensure that
other factors such a logistics are not unduly complicated by requiring
different types of transmitting and receiving antennas. This may be
particularly true in transportable applications.

With broadband radiating structures of the log-periodic type when imaged
above the ground, directive gain figures ranging from 10 to 15 dB are
obtained. The effect of ground depends on various factors such as the
electrical constant of the ground, height of the antenna above ground,
antenna polarization, geometry of the ground screen and take-off angle.
For the case of horizontally polarized antennas, the effect of ground is
usually negligible for heights greater than about 0.2 wavelengths. It
is difficult to generalize on the effect finite ground conductivity has
upon the gain of vertically polarized antennas, however, a few general
remarks can be made. For a vertically polarized antenna of the
quarter-wave monopole type, a ground screen is required to provide a low
loss ground return path to the current at the feed point and to provide
a good reflection plane for radiation at angles close (in the order to
10°) to the horizon. In the case of vertically polarized antennas of the
half-wave dipole type, the requirement for a ground screen is to
provide radiation at low take-off angles. These facts must be kept in
mind when considering antenna types for specific applications.

Feed point Impedance. From the standpoint of systems
requirements, most cases involve either 50 ohms coaxial cable or 600
ohms balanced line, although at times 300 ohm balanced line is used for
high power transmitting systems. Unfortunately, most of the practical
and economical log-periodic antennas do not have input impedance values
that are either 50 ohms coaxial or 600 ohms balanced, it is, therefore,
necessary to provide baluns or transformers. Availability of these
devices has allowed the antenna designer the freedom to design with
relative abandon of input impedance level (provided VSWR with respect to
this level does not exceed a predetermined point) in order to optimize
the structure from other electrical and mechanical considerations.

VSWR. For the applications discussed herein, VSWR of the antenna
is of importance primarily to the transmitting case. A low VSWR is
essential for low power solid state transmitters whose output stage
design is such that in order to keep voltages and/or currents to a
minimum, the output automatically reduces in the event that a
predetermined terminal VSWR is exceeded, it also facilitates the task of
the tuning mechanisms in a fast-tuned transmitter. A low VSWR also
permits near optimum utilization of the minimum size transmission line
for the applied power and reduces losses along the line. A VSWR of 2:1
with respect to antenna feedpoint impedance is typical for broadband
radiating structures of the log-periodic, spiral or conical types.

Power Handling Capability. A trend exists towards increasing the
transmitted power level in HF systems. This trend is not entirely
unjustified in view of the usage of multichannel or multimode
transmission; the overall power capability of the transmission system
must be sufficient to allow adequate power levels per channel. Another
reason for operating at high power levels is to increase reliability,
particularly in an emergency situation. Aside from cost, the main
detraction from high power operation is the problem of interference;
improvements in radiation pattern characteristics and in frequency
management has, however, alleviated this situation.

Wind Speed and Ice Loading. On the matter of mechanical
specification realism in regard to wind and ice loading is in keeping
costs to a reasonable level. This is due, of course, to the fact that
loading is proportional to the square of the wind velocity, and icing
not only increases the weight of the structure, but also adds wind drag
area without increasing strength. Many of the larger log-periodic arrays
are capable of withstanding 1 cm radial ice simultaneously with winds
of 160 kph.

Land Area and Tower Height. Undoubtedly, an important factor in
use fulness of log-periodic antennas in HF communication systems is the
reduced requirement for land, compared to that of the rhombic type
antenna. It is true that large rhombic antennas have greater maximum
gain (by about 5 dB to 10 dB) than practical log-periodic antenna
designs; however, it must also be recognized that in the case of
rhombics the maximum gain is orientated in the proper projection over a
relatively narrow frequency range. Real estate is a major cost factor in
HF antenna farms and the availability of multimode antennas, such as
the SPIRA-CONE, help to minimize these costs by reducing the number of
antennas required to accomplish a given operational requirement,
especially when considering diverse range requirements of ship/shore,
ground/air communications.

Tower height requirements are determined from specifications of low
frequency cut-off and for horizontally polarized antennas, the
additional specification of take-off angle. For high performance
vertically polarized log-periodic arrays of the half-wave dipole type,
the last radiating element is half-wavelength at the lowest operating
frequency, tower height is about 0.7 wavelengths at the lowest operating
frequency; for antennas of the quarter wave monopole type this figure
is halved.

ANTENNA SITING

A usual problem in planning a HF communication system is the matter of
antenna siting and to assist in this matter a few comments and useful
references will be given. Firstly, it may be helpful to note certain
basic distinctions between horizontally and vertically polarized
antennas as each has to be treated a little differently.

A vertically polarized antenna, for example, may require a ground
screen, and the nature of the terrain immediately around the antenna
under which the ground screen must be buried may influence the exact
placement and manner of installation of the antenna. The signal in the
direction of interest will always be a combination of direct radiation
from the antenna and energy reflected from the ground in front of it
even beyond the ground screen, if one is used. Therefore, to ensure
having a well directed beam in space, it is necessary that a fairly
smooth area be available for a reasonable distance in front of the
antenna. This distance may be as short as a few hundred metres for
fairly high angle radiation or as long as a few thousand metres when
low-angle radiation is of greatest interest. The distant terrain, up to
several kilometres from the site, must also be considered, and the usual
rule of thumb is that the angular elevation of the top of a distant
range of hills in the direction of propagation should not be greater
than one-half of the nominal take-off angle of the signal path. This may
influence the selection of the site, although economic or political
factors are probably the dominant factor.

Man-made objects near the antenna must also be considered, and for
vertically polarized antennas, vertical objects such as steel towers and
the like will naturally have the greatest potential for radiation
pattern distortion. There are no general rules of thumb for required
separation, but when the question arises a reasonable estimate can
usually be made by estimating the mutual and self impedances of the
elements involved and the currents which might flow in the parasitic
radiator. The required separation is governed by the type of
interference which is of greatest concern. Depending on the individual
case, antenna VSWR, transmitter interaction, transmitter-receiver
coupling, or radiation pattern distortion may set the criteria for
clearance around the antenna.

In the case of horizontally polarized antennas, the immediate area
underneath the antenna has very little effect on the antenna
performance, particularly on the antenna impedance, and little grading
is required. However, the reflection area in front of the antenna is
still important, and the terrain must be relatively smooth for distances
of a few thousand metres, depending on the frequency and the elevation
angle of interest. Man-made objects will also be of concern; here the
problem would primarily involve horizontal conducting objects such as
power lines rather than vertical conducting objects. The establishment
of minimum separations would be the same as was mentioned for vertical
antennas.

A useful reference on siting of radio terminals is National Bureau of
Standards Technical Note 139, dated April 1962, prepared by William F.
Utlant.

The matter of relative spacing and orientation between antennas is
important because it affects land area requirements and electrical
performance. When transmitting, an antenna will transfer some of its
radiated energy to any other antenna in relatively close proximity, and
this transferred energy will affect the performance of the other
antenna.

Ideally HF antennas of unlike functions, transmitting and receiving,
should be separated by several kilometres (some authorities stipulate a
minimum distance of 24 kilometres) if the latter's performance is not to
be degraded due to interference from the former. This interference can
be caused by adjacent channel operation, harmonics, keying transients
and parasitic oscillations. Also, cross modulation products can be
generated in HF pre-amplifiers and receivers by strong RF fields, even
though normal receiving frequencies are widely separated from the
frequencies of such fields.

The amount of energy coupled between antennas of like functions, all
receiving or all transmitting, can be calculated accurately by solving
the fundamental electromagnetic equations using a comprehensive antenna
analysis computer program. However, the following criteria serve as a
useful reference for planning purposes. All distances, unless otherwise
noted, are based on the antenna's lowest design frequency. The larger of
the two distances in each case is used as the spacing distance. The
points of measurement are between the reference points listed for each
type of antenna.

Horizontal Log Periodic Antenna - space two wavelengths from
the main lobe and one wavelength outside the main lobe, measured from
the main supporting structure (midway between supporting structures for
two tower configurations).

Vertical Log Periodic Antenna - spacing requirements are the same as for horizontal log periodic antennas.

Rotatable Log Periodic Antenna - space two wavelengths
from horizontally polarized antennas. The separation requirement from a
vertically polarized antenna is determined by the spacing requirement of
the vertical antenna. In all cases, spacing must not be less than 45
metres.